The present invention generally relates to lighting systems and related technologies. More particularly, this invention relates to phosphors and coating systems utilized by fluorescent lamps to generate visible light.
Fluorescent lamps have been in use and commercialization since the 1930s. More recently, both consumers and producers have voiced increased desire for energy efficiency and environmental impact of products, spanning all industries, including the lighting industry. As such, fluorescent lights have seen an increase in usage due to their increased energy efficiency when compared to conventional incandescent lights. Significant effort and research have been made in the interest of improving fluorescent light lumen output without increasing power requirements or significantly increasing material costs.
A nonlimiting example of a fluorescent lamp 10 is schematically represented in FIG. 1. The lamp 10 is represented as having a sealed tube comprising a transparent (for example, glass) envelope or shell 12 enclosing an interior chamber 14. The chamber 14 is preferably at very low pressure, for example, around 0.3% atmospheric pressure, and contains a gas mixture having at least one constituent that can be ionized to generate radiation that includes ultraviolet (UV) wavelengths. According to the current state of the art, such a gas mixture typically includes one or more inert gases (for example, argon) or a mixture of one or more inert gases and other gases at a low pressure, along with a small quantity of mercury vapor. Electrodes 16 inside the chamber 14 are electrically connected to electrical contact pins 18 that extend from oppositely-disposed bases 20 of the lamp 10. When the contact pins 18 are connected to a power source, the applied voltage causes current to flow through the electrodes 16 and electrons to migrate from one electrode 16 to the other electrode 16 at the other end of the chamber 14. In the process, this energy converts a small amount of the liquid mercury from the liquid state to a charged (ionized) gaseous (vapor) state. The electrons and charged gas molecules move through the chamber 14, occasionally colliding with and exciting the gaseous mercury molecules, raising the energy level of the electrons in the mercury atoms. In order to return to their original energy level, the electrons release photons.
Due to the arrangement of electrons in mercury atoms, most of the photons released by these electrons are in the ultraviolet (UV) wavelengths. This is not visible light, and as such for the lamp 10 to emit visible light these photons must be converted to a visible light wavelength. Such a conversion can be performed by a coating 22 disposed at the interior surface of the transparent shell 12. The coating 22 often contains phosphor powders and is separated from the shell 12 by a UV-reflecting barrier layer 24 of, for example, alumina (Al2O3). The UV wavelengths emitted by the ionized mercury vapor are absorbed by the phosphor composition within the coating 22, resulting in excitation of the phosphor composition to produce visible light that is emitted through the shell 12. More particularly, when electrons of the phosphor are struck by photons, the electrons become excited to a higher energy level and emit a photon to return to their original energy level. The emitted photon has less energy than the impinging photon and is in the visible light spectrum to provide the lighting function of the lamp 10. The color and luminosity of the lamp 10 are largely the result of the phosphor or phosphors used in the coating 22.
The mercury in low pressure fluorescent lamps predominantly emits UV radiation having a wavelength of 254 nm, and to a lesser extent a wavelength of 185 nm. As used herein, “predominantly” and “predominant” mean that something contains more of one constituent (the “predominant constituent”), e.g., by weight, volume, molar, or other quantitative percent, than any other individual constituent. As these terms are used herein in relation to radiation, “predominantly” and “predominant” signify a wavelength that is more prevalent in a band of radiation than any other individual wavelength. Some estimates are that roughly 90% of UV radiation generated by low pressure fluorescent lamps is at the predominant 254 nm wavelength, with the balance (roughly 10%) being the 185 nm wavelength. Both of these wavelengths fall within a wavelength range known as ultraviolet subtype C. Phosphors used in low pressure mercury lamps are typically excited by different ranges of wavelengths encompassing the primary wavelength (254 nm) to absorb as much UV radiation as possible. Aluminate phosphors and particularly barium-magnesium-aluminates (BAM) are examples of phosphors widely used as components of phosphor blends in fluorescent lamps intended for white light generation. BAM phosphors having the general formula BaMg2Al16O27:Eu2+ absorb the predominant 254 nm wavelength as well as the secondary 185 nm wavelength of UV radiation emitted by low pressure mercury lamps, and utilize europium ions as an activator to impart the desired phosphor property. As known in the art, the terms “activator” and “activated” refer to the effect that doping with dopants, for example, europium, manganese, cerium, terbium, etc., have with respect to the luminescence of a phosphor. For example, whereas BAM phosphor activated by divalent europium (BaMg2Al16O27:Eu2+) primarily absorbs UV radiation at the 254 nm wavelength to emit blue visible light, barium-magnesium-aluminate can be co-activated by divalent europium and manganese (BaMg2Al16O27:Eu2+,Mn2+) to primarily absorbs UV radiation at the 254 nm wavelength to emit blue-green visible light. Both of these phosphors also generate visible light as a result of absorption of the secondary 185 nm wavelength, but their visible light generation resulting from their absorption of this wavelength is much less efficient than for the 254 nm wavelength, with the result that the proportional amount of lumens attributable to the absorption of the 254 nm wavelength is much greater than proportional amount of the 254 nm wavelength in the UV radiation.
The efficiency and effectiveness of a fluorescent lamp and its coating system depend on what particular phosphors are used and what wavelengths of UV radiation are absorbed. A great deal of effort has been made to increase the luminosity of fluorescent lamps without increasing the power requirements, thereby increasing their efficiencies. Currently, the most effective solution to increase luminosity involves the use of rare earth-containing phosphor compositions, including rare earth compounds and transition metal compounds that serve as host materials doped with one or more rare earth activators that, for example, prolong emission time. As nonlimiting examples, in addition to the aforementioned europium-activated BAM phosphor (BaMg2Al16O27:Eu2+) utilized as a blue-emitting phosphor, phosphor compositions have been employed that contain a controlled mixture of, cerium-and terbium-coactivated lanthanum phosphate phosphor (LaPO4:Ce3+,Tb3+; LAP) as a green-emitting phosphor, and europium-activated yttrium oxide phosphor (Y2O3:Eu3+; YOE or YEO) as a red-emitting phosphor, mixed in appropriate ratios.
Rare earth elements are generally considered to include seventeen chemical elements, namely, the fifteen lanthanides, scandium, and yttrium. Though fairly abundant, rare earth elements are becoming increasingly prohibitive to procure and use. Demand for these materials, which are also used in medical technology, wind turbines, hybrid automobiles, TVs, smart phones, and computers, as well as other industries, has significantly increased as these technologies have grown. In addition, rare earth materials may occur as mixtures of various rare earth compounds and in low concentrations. These compounds must be mined, concentrated into oxides, and finally separated into individual rare earth elements or compounds suitable for the intended application. Finally, the majority of the world's rare earth reserves are currently believed to be in limited locations, leading to concerns over pricing and supply stability. As such, the use of rare earth elements has or may become cost-prohibitive, and effective alternative solutions are being sought.
One possible alternative is the integration of additional or alternative phosphor compounds into phosphor blends to improve the latter's efficiency and/or lumen output. Preferred phosphor compounds would be commonly occurring or easily produced in order to improve cost and production factors. In addition, it would be desirable to improve the overall efficiency of a lamp by promoting the ability of the phosphor blends to absorb and be excited by UV radiation wavelengths generated by low pressure mercury lamps.